1. Field of the Invention
The present invention relates to an adhesive polypropylene composition useful as an adhesive and more particularly, to a polypropylene composition useful as an adhesive between a polypropylene resin layer and an ethylene/vinyl alcohol copolymer layer or a polyamide resin layer.
2. Description of the Related Art
Laminates comprising a polypropylene resin layer and an ethylene/vinyl alcohol copolymer layer, or a polypropylene resin layer and a polyamide resin layer are low in oxygen permeability and moisture vapor permeability, and therefore are widely used, for example, for containers for foods and medicines.
However, since adhesion between a polypropylene resin layer and an ethylene/vinyl alcohol copolymer layer or a polyamide resin layer is poor, a molding method is generally employed wherein an adhesive layer is placed between the resin layers by co-extrusion.
It is well known that polyolefin resins graft-modified with an unsaturated carboxylic acid or derivative thereof may be used as a bonding layer (see for example Japanese Patent Application Laid-Open Nos. 5527/1973, 98784/1976, and 26548/1977).
It is also known that the bonding strength between the resin layers may be improved by adding a low-density polyethylene to the graft-modified polyolefin resin (see for example Japanese Patent Publication No. 40113/1979), adding a high-density polyethylene to the graft-modified polyolefin resin (see for example Japanese Patent Publication No. 36586/1984), adding a hydrocarbon elastomer to the graft-modified polyolefin resin (see for example Japanese Patent Publication No. 40112/1979). It is also known to use a mixture comprising a propylene/ethylene block copolymer, an ethylene/.alpha.-olefin random copolymer, an unsaturated carboxylic acid or its derivative and an organic peroxide (see for example Japanese Patent Application Laid-Open No. 41205/1981). However, such a method results in an odor and inferior coloring due to unreacted monomers.
When compositions prepared by adding a low-density polyethylene, a high-density polyethylene, or a hydrocarbon elastomer to a graft-modified polyolefin resin are employed as an adhesive layer, the performance is inadequate. For example, in the case wherein a laminate made up of a polypropylene resin layer and an ethylene/vinyl alcohol copolymer layer or a polyamide layer is blow molded into a bottle, the thin section of the product is low in adhesion strength. Accordingly, further improvement is required.
Further, the above prior art compositions have low softening points and as a result the adhesion strength under high temperatures is low.